Heterodyne laser instantaneous frequency measurement system

ABSTRACT

A heterodyne laser instantaneous frequency measurement system is disclosed. The system utilizes heterodyning of a pulsed laser beam with a continuous wave laser beam to form a beat signal. The beat signal is processed by a controller or computer which determines both the average frequency of the laser pulse and any changes or chirp of the frequency during the pulse.

FIELD OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. W-7405-ENG-48 between the United States Department ofEnergy and the University of California for the operation of LawrenceLivermore National Laboratory.

This is a continuation of application Ser. No. 911,023 filed Sept. 24,1986 now U.S. Pat. No. 4,798,467.

BACKGROUND OF THE INVENTION

The present invention relates to a heterodyne laser instantaneousfrequency measurement system.

In one preferred embodiment, the present invention is intended for usein an atomic vapor laser isotope separation (AVLIS) process whichutilizes pulsed laser beams for photoionizing an atomic vapor. Optimumoperation of an AVLIS process requires precise control of the frequencyof the pulsed lasers used in the process. The isotope frequency shiftand hyperfine splittings of optical transitions in heavy atoms aretypically between 0.1 and 10 GHz. Hyperfine spectral features can haveline widths of less than 10 MHz in atomic beams to more than one GHz inthermal sources. The efficiency of such a process is often dependentupon accurately placing the laser frequency on the center line of thehyperfine spectral features, requiring an absolute frequency error ofless than 5 MHz. For 500 THz red light, this corresponds to a maximumfractional frequency error of one part in 10⁸.

In view of the above background, it would clearly be desirable toprovide for a frequency measurement system which is capable of providingan accurate indication of the instantaneous frequency of a pulsed laserbeam.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an instantaneousfrequency measurement system.

It is a more particular object of the present invention to provide aheterodyne laser instantaneous frequency measurement system which candetermine the instantaneous frequency of a laser pulse.

It is a further object of the present invention to provide a systemwhich can determine both the average frequency of a laser pulse and anychanges or "chirp"of the frequency during the pulse.

Briefly, the laser beam frequency diagnostic system includes a firstpulsed laser for generating a first pulsed laser beam, the instantaneousfrequency of which is to be determined. The system further includes asecond continuous wave (CW) reference laser for generating a secondlaser beam having a predetermined frequency range.

The system further includes mean for heterodyning the first and secondlaser beams to form a beat signal representative of the differencebetween said first and second frequencies.

The system also includes means for processing the beat signal todetermine the instantaneous frequency of said first laser beam.

Additional objects, advantages and novel features of the presentinvention will be set forth in part in the description which follows andin part become apparent to those skilled in the art upon examination ofthe following or may be learned by practice of the invention. Theobjects, advantages and features of the invention may be realized andattained by means of the instrumentalities and combinations particularlypointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate an embodiment of the invention and,together with the following detailed description, serve to explain theprinciples of the invention.

FIG. 1 depicts a block diagram of a pulsed laser heterodyneinstantaneous frequency measurement system according to the presentinvention.

FIG. 2A depicts a timing waveform of pulsed laser amplitude andheterodyne beat signal data.

FIG. 2B depicts a timing waveform of relative phase variation of apulsed laser from a constant laser frequency.

FIG. 2C depicts a timing waveform of an instantaneous optical differencefrequency between a pulsed laser and a CW reference laser.

FIG. 3 depicts a diagram of a microwave mixer down conversion of anoptical heterodyne beat signal according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference will now be made in detail to a preferred embodiment of theinvention, an example of which is illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thatpreferred embodiment, it will be understood that it is not intended tolimit the invention to that embodiment. On the contrary, it is intendedto cover alternatives, modifications and equivalents as may be includedwithin the spirit and scope of the invention as defined by the appendedclaims.

Referring now to FIG. 1, a block diagram of a laser heterodyneinstantaneous frequency diagnostic system according to the presentinvention is depicted.

The laser heterodyne instantaneous frequency diagnostic system 10 canmeasure the absolute frequency of pulsed lasers as well as frequencyvariation of the laser during the pulse (as short as 10-100 nanosecondspulse length). The diagnostic system 10 digitally records the heterodynebeat signal of the pulsed laser and a CW reference laser, which is at anappropriate known offset frequency. A computer than analyzes this datato calculate and plot instantaneous frequency versus time during thelaser pulse. The system 10 measures absolute frequency offsets with anaccuracy of better than 5 MHz and can detect frequency variations ofabout this same magnitude which occur in intervals as small as about 3nanoseconds during the pulse.

Referring to the system 10 depicted in FIG. 1, a dye master oscillator12 is suitably pumped by a copper vapor laser 14 in order to generate(or bring about the lasing) of a first laser beam 18, the frequency ofwhich is to be calculated or determined by the system 10.

The pulsed laser beam 18 is optically coupled through a 2.0 ND (NeutralDensity) filter to a single mode fiber 16, which is connected to asuitable coupler 20 (such as a Gould fiber coupler).

The copper vapor laser 14 also provides a trigger output on lead 68 fortriggering purposes for digitizer 70, as will be described.

The first laser beam is proportionally split by coupler 20, 91% of whichis input to detector 50, and 9% of which is input to combiner 36.

The system 10 also includes a continuous wave (CW) laser 30 having apredetermined offset frequency. The laser beam from CW laser 30 is inputthrough a polarization rotator 32 to single mode fiber coupler 36.

The pulsed laser beams from pulsed laser 12 and the CW laser beam fromlaser 30 are combined onto a single mode fiber 40. The polarizationrotator 32 is adjusted to align the two laser beam polarization at thedetector 52 to achieve a maximum heterodyne beat signal.

In one embodiment, a 91% portion of a pulsed laser beam is input viafiber 24 to intensity pulse detector 50 and the combined pulsed/CW laserfrom fiber 20 is input to heterodyne pulse detector 52.

The outputs of detectors 50, 52 are input through signal combiner 64 onlead 66 to digitizer 70.

In FIG. 1, a silicon diode detector 52 with a linear relationshipbetween incident light intensity and photo-current increases theaccuracy and time resolution of the instantaneous frequencycalculations. The optical heterodyne difference-frequency is generallybelow 1 GHZ, so extremely high detector frequency response is not asimportant as the linearity of detector output current versus opticalintensity. The heterodyne beat signal from detector 52 and the intensityamplitude waveform of the pulsed laser from detector 50 are bothdigitally recorded for the individual laser pulse under test, using adigitizer 70 (desirably a very fast Tektronix 7912AD TransientDigitizer). Since only one signal channel is available for real timeanalog to digital conversion by the transient digitizer 70, theheterodyne beat signal is delayed by either an electrical transmissionor optical delay line 42 and then combined on the same signal line 66 asthe pulsed laser amplitude waveform before input to the transientdigitizer 70. If desired, a separate digitizer could be provided for thetest signal and the heterodyne beat signal.

After analog to digital conversion by the transient digitizer 70, thedigital data is transferred by a standard GPIB bus 74 to a computer (orcontroller) 80. The computer 80 then analyzes the combined beat signaland amplitude signal data to calculate the instantaneous opticaldifference frequency between the pulsed laser 12 and the CW laserreference 30.

The heterodyne data is first normalized using the pulsed laser amplitudedata. Relative phase between the actual beat signal and an assumedconstant frequency average beat signal is then calculated for theduration of the pulse. Finally, the derivative of the relative phasesignal versus time is calculated. This result may be added to theassumed heterodyne frequency difference as well as the absolutefrequency of the CW laser reference to yield the instantaneous opticalfrequency versus time. Absolute pulsed laser frequency and pulsed laserfrequency changes which occur within a pulse are therefore obtained.Pulse to pulse frequency variations are quantified by repeating themeasurement on additional laser pulses.

A plot obtained using the optical heterodyne system of the"instantaneous optical frequency" variation of a 20 nanosecond durationlaser pulse appears in FIG. 2C. FIG. 2A shows the combined pulsed laseramplitude and heterodyne beat signal data that was captured by thetransient digitizer 70 to obtain the plot of FIG. 2C. In FIG. 2A, curve84 represents a single pulse from intensity pulse detector 50 of FIG. 1,and curve 86 represents the delayed heterodyne beat signal fromheterodyne detector 52 of FIG. 1. The heterodyne signal is delayed byabout 42 ns. FIG. 2B is a plot of an intermediate computer resultshowing the relative phase variation of the pulsed laser 12 with respectto the CW reference laser 30, as indicated by curve 88. FIG. 2C, whichdepicts the instantaneous optical frequency of the pulsed laser 12 bycurve 90, is obtained by taking the derivative of FIG. 2B. Anappropriate time response filter is added for signal to noiseimprovement. Appendix A represents an implementation of the computeralgorithms which processes the digitized heterodyne beat and pulsedlaser intensity signal data to yield the instantaneous frequency plot.This example uses a commercially available software package calledMathgraf.

The state of the art of transient digitizer bandwidths dictates thepractical upper limit of the heterodyne beat frequency. This iscurrently about 6 GHz. To extend the maximum allowable frequencydifference between a processed pulsed laser and a CW laser, an externalmixer may be used with a microwave local oscillator to down-convert thelaser beat signal frequency to lie within the bandwidth of the transientdigitizer, as shown in FIG. 3.

In FIG. 3, the outputs from a reference laser 02 and a pulse laser 100are coupled in fiber coupler 06 to heterodyne detector 110. The beatsignal from detector 110 is filtered through high pass filter 114 tomicrowave mixer 120, where the signal is mixed with the output ofmicrowave local oscillator 116. The output of mixer 120 through 0-1 GHzamplifier 124 is then input to the transient digitizer 70 of FIG. 1.

This allows locking of the reference laser 102 to available spectralreference lines which may be at frequency offsets beyond the 6 GHzfrequency limitation of the transient digitizer 70 of FIG. 1. A secondadvantage of this approach is related to an improvement in signal tonoise arising from a source of system noise typical to tunable CW dyelasers called spurious homodyne signals. These spurious beat signalsarise from intermodulation products between the low level secondarymodes of the reference laser 102 and the main mode, and are thereforecalled homodyne signal since only the reference laser 102 is requiredfor their generation. These signals appear with random phase in the timedomain and are therefore an important source of noise on the transientdigitizer input signal. Since the secondary modes fall off drasticallyat high frequencies offsets from the main mode, their effect may beeliminated by using a high pass filter 114 between the heterodynedetector 110 and the external microwave mixer 120.

The use of fiber-optics to couple input light to the optical detectorsimplifies measurements in the large laser systems typically used inAVLIS applications. Measurements may be performed at several points in acomplex master oscillator, multiple amplifier laser system while using asingle instrument in one location. Utilizing only one optical detector,one reference laser, one transient digitizer and time multiplexing ofthe optical signals under test greatly reduces system complexity. Thefiber-optics allows electrical isolation of the detector, reducing manyof the electromagnetic interference problem associated with recordingthe comparatively low signal levels from the optical detector in theimmediate presence of the extremely large pulsed power waveformsrequired by the pulsed lasers.

Heterodyne techniques have been applied to the measurement of pulsedlaser optical frequency. A pulsed laser optical frequency diagnosticsystem has been developed which allows comparison of the absolute laserfrequency of individual laser pulses or detection of changes of laserfrequency within a single laser pulse. The use of fiber-optics in thissystem minimizes complexity and solves many optical alignment andelectromagnetic interference problems.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously many modifications and variations are possiblein light of the above teaching. The present embodiment was chosen anddescribed in order to best explain the principles of the invention andits practical applications to thereby enable others skilled in the artto best utilize the invention and various embodiments and with variousmodifications as are suited to the particular use contemplated. Forexample, a pulsed reference laser could be utilized where the pulsedreference laser has a longer duration than the pulse under test. Also,the pulsed test laser and pulsed reference laser should be synchronousto one another. It is intended, therefore, that the scope of theinvention be defined by the claims attached hereto.

What is claimed is:
 1. A laser beam frequency diagnostic systemcomprisinga first pulsed laser for generating a first short timeduration pulsed laser beam having a pulse length duration ofapproximately 10-100 nanoseconds, the instantaneous frequency of whichis to be determined, a second reference laser for generating a secondlaser beam having a predetermined frequency, means for heterodyning saidfirst and second laser beams to form a beat signal representative of thedifference between said first and second frequencies, and means forprocessing said beat signal with respect to time to determine theinstantaneous frequency of said first laser beam.
 2. A system as inclaim 1 wherein said second reference laser is a continuous wave (CW)reference laser.
 3. A system as in claim 1 wherein said second referencelaser is a pulsed laser.
 4. A system as in claim 1 including mixer meansfor converting said beat signal down to a lower frequency.
 5. A systemas in claim 1 including means for time multiplexing said beat signal andsaid pulsed laser intensity signal.
 6. A laser beam frequency diagnosticsystem for determining the instantaneous frequency of a short timeduration pulsed laser beam having a pulse length duration ofapproximately 10-100 nanoseconds, said system comprisinga continuouswave (CW) reference laser for generating a CW laser beam having apredetermined frequency, means for heterodyning said pulsed laser beamand said CW laser beam to form a beat signal representative of thedifference between said first and second frequencies, and means forprocessing said beat signal with respect to time to determine theinstantaneous frequency of said first laser beam.
 7. In a laser beamfrequency diagnostic system, the method comprising the stepsofgenerating a first short time duration pulsed laser beam having apulse length duration of approximately 10-100 nanoseconds, theinstantaneous frequency of which is to be determined, generating asecond continuous laser beam having a predetermined frequency,heterodyning said first and second laser beams to form a beat signalrepresentative of the difference between said first and secondfrequencies, and processing said beat signal with respect to time todetermine the instantaneous frequency of said first laser beam.